Diversity of mixed-function oxidases in human placenta microsomes, monocytes and lymphocytes was investigated using monoclonal antibodies to 3-methylcholanthrene (MC)-induced rat liver cytochrome P-450. Monoclonal antibodies specific to MC-induced P-450 were obtained by hybridizing mouse myeloma cells with spleen cells of mice immunized with MC-induced rat liver cytochrome P-450. Monoclonal antibodies to rat liver cytochrome P-450 not only inhibited the aryl hydrocarbon hydroxylase (AHH) of MC-induced rat liver microsomes but also inhibited AHH of human placenta and lymphocytes, 90% and 50%, respectively. The inhibitory effects of monoclonal antibodies of AHH of monocytes was negligible. 7-Ethoxycoumarin deethylase activity of human placenta and lymphocytes was inhibited 50%. No inhibition of these activities was observed with monocytes. The cross-reactivity of monoclonal antibodies to rat MC-induced cytochrome P-450 may be useful for studying the diversity and multiplicity of cytochromes P-450.